In the cable industry, it is well known that optical fibers must not suffer mechanical stress if both their transmission performance and their lifetime are not to be degraded.
To limit traction stresses on optical fibers while cables are being laid, or during temperature variations which cause expansion or retraction of the optical fibers, it is known to use reinforcing members which may be central or peripheral. In the case of a central reinforcing member it is generally necessary to twist the optical fibers or the elements containing the optical fibers. This slows down the rate of production and increases the cost of the cable obtained.
To avoid the twisting operation, it is known to make an optical fiber cable by enclosing a bundle of optical fibers in an outer sheath having reinforcing members embedded longitudinally therein. When a cable of this type is subjected to bending, the reinforcing members which are located on the outside of the bend relative to the middle of the cable, are lengthened by an amount that increases with increasing diameter of the cable. This results in resistance of the cable to bending and in the risk of the reinforcing members breaking if the force applied to the cable is greater than a force which causes the outermost reinforcing members to lengthen to breaking point.
To avoid that problem, it has been envisaged in document DE-A-38 15565 to dispose only two diametrally opposite reinforcing members, or even to dispose diametrally opposite reinforcing members in groups including three spaced-apart reinforcing members or four reinforcing members arranged in a housing filled with a material having a low coefficient of friction to enable the reinforcing members to move relative to the sheath. However, in the case of spaced-apart reinforcing members it is not possible to bend the cable about an axis parallel to the line interconnecting the reinforcing members without subjecting the reinforcing members to differential elongation which risks causing them to break. Furthermore, in the case of reinforcing members disposed in a housing, the outer sheath does not participate in the traction force and it is thus necessary to provide a greater number of reinforcing members. In both cases the sheath in which the reinforcing members are embedded must itself be relatively thick, thereby reducing the bending capacity and reduces the compactness of the cable.